Project Summary Cannabis use poses an important public health problem among adolescents and emerging adults (A/EAs), given increasing perceived acceptability and prevalence of cannabis use. Cannabis interventions in schools show some promise, although half of EAs are not enrolled in school, and drug use among adolescents is associated with school absenteeism, limiting the relevance of school-based interventions. Brief interventions (BIs) for cannabis in healthcare settings such as Emergency Departments (EDs) have also shown promise, but effect sizes are relatively small and dissipate over follow-up. These interventions need significant updates to both extend the effects and address recent changes in perceptions, access, and route of administration (e.g., edibles, vape pens). In recent years, engagement with social media (SM) has dramatically increased, with the majority of A/EAs utilizing several SM platforms daily. High levels of engagement with SM content provide a dynamic platform to go beyond ED-based BIs and engage A/EAs residing in socio-economically disadvantaged communities with cannabis reduction interventions. The specific aims of the proposed study are to: 1) iteratively develop and focus test a SM-based intervention for A/EAs' cannabis use, and 2) test the preliminary efficacy of the intervention versus a control condition on cannabis outcomes. Secondary aims include identifying key mediators/moderators of outcomes (e.g., sex, motives). We will use an iterative process informed by prior ED-based BIs to develop and refine an ED-initiated SM intervention for cannabis use among A/EAs. This intervention will include dynamic SM-delivered content for 4 weeks post-ED visit using private SM- messaging delivered by peer coaches via a popular SM platform (i.e., Snapchat). Based on prior work, we will develop engaging, user-generated content using crowdsourcing via M-Turk paired with iterative focus testing. The intervention will harness peers as health coaches to be positive influences, which is scalable given the availability of student trainees and will focus on ?up-stream? factors for cannabis use such as motives for use (e.g., stress/coping, social), blending evidenced based approaches for cannabis risk reduction (e.g., motivational interviewing, cognitive-behavioral skills). The intervention will be tested in a pilot randomized controlled trial versus an attention-placebo control (usual care+4-week social media entertainment/news condition) among 16-24 year-old A/EAs who use cannabis at least weekly. Outcomes will be measured at a 1- month post-test and 3-month follow-up, with biological verification of drug use. As opposed to one-session BIs and static web-based interventions with limited ?shelf-life?, the development of an adaptable, scalable and efficacious intervention for cannabis use among A/EAs, meeting them where they are at, on SM, is a critical next step in public health efforts to reduce cannabis use/consequences. This study could have significant impact by altering the cannabis use trajectories of A/EAs, preventing health and social consequences and the development of cannabis use disorders, as well as informing future SM interventions for behavioral health.